Emerging infectious diseases have been increasing in incidence and are a key threat to wildlife and human health. Influenza naturally infects wild birds all around the world, although they usually do not become ill. The virus is very contagious, however, and it can become a problem when the virus is transmitted to domesticated birds, such as chickens, ducks, or turkeys, because domesticated poultry can succumb to illness and death from influenza. Since 1997, H5N1 infections in birds have spread, initially throughout Asia. Then as birds traveled along their migratory routes, H5N1 dispersed to Russia and Europe, and later to countries in the Middle East and on the African continent. Wild and domestic birds are recognized as the reservoirs of most influenza A viruses. Although the extent to which birds are involved in the emergence and global spread of novel, pandemic human strains remains debated, even the most recent pandemic strain, H1N1, contains several segments that most likely originated in birds. Thus, the dynamics of influenza infections among birds and mammals (including humans) are intimately linked.